TOKYO (AP) — The U.S Navy said Thursday it has suspended its search for a Japanese air force F-35A stealth fighter that crashed off Japan’s coast last month, after the allies scrambled to locate the aircraft to protect its military secrets.

The pilot is still missing, and Defense Minister Takeshi Iwaya said Japan will continue its search.

The Navy said in a statement that it was withdrawing after a salvage vehicle, CURV 21, found debris from the aircraft. It crashed in the Pacific off the eastern coast of Aomori in northern Japan during a night training flight on April 9. It went missing about half an hour after taking off from Misawa air base with three other F-35As and about a minute after the pilot abruptly ordered an end to the training exercise.

The Navy was unusually quick in the salvage effort amid air and maritime activity in the region by China and Russia. It dispatched the guided-missile destroyer USS Stethem and P-8A Poseidon surveillance aircraft to join a Japanese search and rescue team. From April 9 to 17, the U.S. search covered more than 5,000 square nautical miles (17,150 square kilometers) before deploying the CURV 21 remotely operated vehicle, which is capable of salvage operations at a depth of up to 20,000 feet (6,000 meters).

Iwaya told the upper house Diplomatic and Defense Affairs Committee that the U.S. salvage vehicle recovered parts of a flight data recorder but the flight data were missing. A joint effort using sonar by the Japanese deep undersea vehicle Kaimei also located and recovered parts of a canopy and other equipment, he said.

Iwaya said Kaimei has also withdrawn, but the surface and underwater search is continuing with the participation of Japanese Air Self-Defense Force vessels as well as a private salvage boat.

“We will continue our search and recovery for the pilot and the aircraft that are still missing, while doing our utmost to determine the cause,” he said.

Findings of the investigation could affect Japan’s F-35 purchase plans, though officials say there has been no change.

Japan started deploying the expensive F-35s last year as part of a plan to bolster its capability to counter potential threats from North Korea and China. Under guidelines approved in December, Prime Minister Shinzo Abe’s government plans to buy 147 F-35s, including 105 F-35As, costing about 10 billion yen ($90 million) each.

Some were assembled by Japanese defense manufacturers under licensing contracts with Lockheed Martin, a plan that has been scrapped because of higher costs. Japan’s government earlier hoped to make the country a regional assembly and maintenance hub for F-35s. The crashed aircraft was the first assembled in Japan by Mitsubishi Heavy Industries.

The crashed jet had previous safety issues and they will be reviewed, Iwaya has said. During a test flight in September 2017, it made a “precautionary landing” due to a problem related to a cooling system. In August 2018, it made another unscheduled landing because of a navigation system problem.

All 12 remaining F-35 jets at the Misawa base remain grounded.

The U.S. Air Force has grounded some F-35A jets in the past after pilots experienced symptoms of hypoxia, or oxygen deficiency.